1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer which forms a multi-color image.
2. Description of Related Art
In a printer or the like for reproducing a full-color image, digital image data R, G, B of red, green and blue of primary colors obtained by reading a document are transformed to data C, M, Y of the complementary colors of cyan, magenta and yellow used for image reproduction.
Therefore, data processing is performed in order to transform the digital data of the three colors, red, green and blue, to data for the three colors for image reproduction.
As to the data processing for a full-color image, following three points have to be taken into consideration: (a) the compatibility of the visibility of black and the saturation of colors, (b) the improvement of color reproducibility, and (c) the compatibility of resolution and smoothness.
As to the first point (a) of the reproduction of black in a full-color image, pure black is hardly reproduced by overlapping cyan, magenta and yellow toners due to the spectral characteristics of the toners. Then, the reproducibility of black is improved by using subtractive mixture of the reproduction colors, cyan, magenta and yellow, and by using black paint. However, in this method, the visibility of black is improved by increasing the degree of black paint, while the saturations of chromatic colors are lowered. For a full-color image, the improvement of the pureness of achromatic colors has to be compatible with the improvement of the saturation of chromatic colors.
Then, the black paint may be proposed to be performed according to the decision of achromatic color or chromatic color. However, the decision with use of the read colors of red, green and blue is liable to err due for example to Moire patterns for a dot document and to errors and noises in a document of flat density and in portions where the hue and the brightness change gradually. Especially, the decision at an edge portion in a image or at a portion where the brightness varies much is liable to be erroneous due to the color, shift of print location or the like, so that the black paint may deteriorate the image adversely. Further, in an image of low saturation, the decision of achromatic color and that of chromatic color may alternate rapidly. Then, because the black paint is performed only for pixels decided to be achromatic colors, the resultant image seems to include random noises.
As to the second point (b), the color reproducibility is affected by the masking correction which is performed to compensate the discrepancies of the characteristics of the filters and of the toners from the ideal characteristics.
The masking correction is usually performed by using linear masking coefficients which are determined in order to minimize the average color differences over the whole color reproduction region. However, the color difference between the original color and the reproduced color is not necessarily minimized in some parts in the color reproduction region and the errors in color reproduction and in gradation may become larger. Then, it is said the secondary masking processing including secondary terms such as DR.sub.2, DG.sub.2, DB.sub.2, DR.cndot.DG, DG.cndot.DB and DB.cndot.DR is better. However, this needs a complicated and larger circuit.
In order to solve this problem, a color image read apparatus disclosed in Japanese patent laid open Publication No. 16,875/1990 has a plurality of masking coefficients each for an important color besides a masking coefficient for an image ordinary as to hues and when a user specifies a hue, the masking coefficient of the hue can be used. However, it is troublesome for a user to specify a hue. Sometimes, a user may specify an erroneous hue. Further, it is not necessarily natural that portions of different characters are intermingled in an image.
As to the third point (c), it is better to change a data processing technique according to the characteristics of an image. Edges have to be emphasized for images such as characters and narrow lines, while half-tone images such as photographs have to be smoothed. Selective control between edge emphasis and smoothing has been carried out in some prior arts by detecting an edge for an input image data in order to discriminate a character/photograph portion for a monochromatic image. For example, the density of the center pixels and its density gradient are obtained at least in the two directions and contrast is emphasized.
However, simple edge emphasis is not necessarily performed well for a full-color image because the image density changes according to the hue and the saturation. For example, when color changes from white to red, edge emphasis may be performed, while when color changes from red to cyan, edge emphasis has not to be performed because the hue changes anomalously at an edge as in color ghost phenomenon. An image of human facial skin is especially affected by the processing according to region discrimination.
In a method disclosed in Japanese Patent laid open Publication No. 171,067/1988, the gradient and the gradient direction of an edge are detected for a center pixel for each read color or for each reproduction color in a color image, and the data of the read color or the reproduction color is corrected according to the detection. However, in this method, because the correction is performed for each color, the edge emphasis is performed even for a hue change. Thus, the hue in an image does not necessarily change naturally.
It has been considered that the image reproducibility may be enhanced if the data processing technique is changed according the characteristics of full-color image. For example, regions for character images and those for half-tone images are designated and the data processing is changed appropriate for each region. In this method, a user has to designates the regions. Further, it is troublesome for a user to designate the characteristics of an image in detail. The discrimination of a specified color is used for example for a color change function provided in a printer to change the specified color to another color. In order to prevent the error of the color discrimination of the specified color, for example a mask processing with use of majority decision for decision results is tried. However, the improvement of the color discrimination is only a little.